Cotton rats previously inoculated with formalin-inactivated RSV were challenged intranasally with live RSV in an attempt to experimentally induce a potentiated RSV disease similar to that observed following administration of formalin-inactivated RSV vaccine to infants 20 years ago. Within 24 hours after infection with RSV cotton rats developed pulmonary lesions that reached a maximum by the fourth day. Histologically the lesions resembled an experimental pulmonary Arthus reaction, although adoptive transfer experiments were not successful in defining their pathogenesis. A similar immunopathologic reaction also developed in cotton rats which were challenged with live type 3 parainfluenza virus following vaccination with formalin-inactivated parainfluenza-3 virus. The action of formalin on RSV and parainfluenza 3 virus appears to be responsible for triggering the immunopathologic reactions, because heat inactivation in the absence of formalin, did not confer on virus the ability to induce immunopathology.